In recent years, hybrid vehicles have drawn public attention due to a demand for low fuel consumption and low emission vehicles. Generally, hybrid vehicles utilize an internal combustion engine and a motor generator as power sources. In some hybrid vehicles, as disclosed in a patent document 1 (i.e., a Japanese Patent No. 3812134), engine power is determined based on a drive power and a battery charge-discharge power so that a battery charge state is maintained within a certain preset range. Drive power is a power applied from a tire to a road surface for a travel of a vehicle. In other hybrid vehicles, a battery charge-discharge amount is determined based on the drive power. As such, by increasing or decreasing the engine power by such a battery charge-discharge amount, the engine is operated at an efficient operating point for improved fuel consumption.
In a hybrid vehicle, in order to effectively improve fuel consumption performance, it is increasingly a requested technique to reduce engine fuel consumption while controlling, to a desired value, an electrical charge-discharge energy balance before and after a travel of a vehicle.
However, in the above-mentioned technique, the electrical charge-discharge energy balance of the battery is not taken into consideration. Therefore, the fuel consumption of the engine may not be reduced while controlling the electrical charge-discharge energy balance of the battery to the desired value. As a result, fuel consumption performance may not improve.